It is well known that such impellers have a central hub which is arranged to be mounted on a motor shaft, with a peripheral crown having a succession of vanes or blades for aspirating air axially through the interior of the crown and to deliver it radially towards the outside, together with a set of arms which join the hub to the peripheral crown. These impellers are generally made from one piece, in a molded synthetic material, which imposes a certain number of limitations as regards to mechanical strength and moldability during A design of the various parts of the impeller. It will easily be understood that the arms are subjected to high mechanical stresses, which have to be taken into account during design and choice of material.